gdb: make use of skip_to_space and skip_spaces

gdb: make use of skip_to_space and skip_spaces

Some late feedback on this commit:

  commit 037d7135de575c9e0c20e9158c105979bfee339c
  Date:   Mon Nov 16 11:36:56 2020 +0000

      gdb: improve command completion for 'print', 'x', and 'display'

Suggested making use of the skip_to_space and skip_spaces helper
functions.  There should be no user visible changes after this commit.

gdb/ChangeLog:

	* printcmd.c (skip_over_slash_fmt): Make use of skip_to_space and
	skip_spaces.